In a conventional solution, as shown in FIG. 2, the gas flowing in a pipe 6A at an upstream pressure Pe is expanded from the upstream pressure Pe to a regulated downstream pressure Pa in an expander-regulator 1A which receives a control signal via a line 4A starting from a segment of pipe 3A situated downstream from the expander-regulator 1A. A meter 2A is disposed between pipe segment 3A and a downstream distribution pipe 7A. The meter 2A meters the volume of gas consumed at the downstream pressure Pa, which pressure depends only on the fixed set point of the regulator 1A that serves to keep said downstream pressure constant. To ensure that the metering is reliable, it is necessary to have good flow conditions in the pipe segment 3A between the expander-regulator 1A and the meter 2A. For this purpose, the segment of pipe 3A must comprise substantial rectilinear lengths L. The high level of disturbance that exists in the vicinity of the release valve of the pressure regulator leads to the gas flowmeter device being placed at a distance from the expander-regulator that corresponds to several times the diameter of the pipe interconnecting these two elements. This gives rise to non-negligible bulk.
In another known solution, as shown in FIG. 3, a gas meter 2B is installed upstream from an expander-regulator 1B, which is provided with a link 4B coming from the downstream pipe 7B to receive information concerning the regulated downstream pressure Pa. Under such circumstances, pipe segment 5B between the meter 2B and the expander-regulator 1B can be relatively short and the maximum flow rating of the meter 2B can be small. In this solution, the meter is not subjected to the disturbances generated by the expander-regulator.
However in solution A (metering downstream from expansion) as in solution B (metering upstream from expansion), the link pipes connecting the inlet of the station or the pressure expander-regulator to the meter, e.g. a spinner meter, generally adds additional disturbances due to the particular shape of the link pipes and to the accessories that may be mounted on the link pipes. The flowmeter can thus receive a flow of gas whose velocity profile is deformed or which has a gas stream that is subject to rotation, which is prejudicial to the quality of metering. In addition, the operating dynamic range of stations fitted in those manners, i.e. the ratio between the maximum flow rate Qmax and the minimum flow Qmin between which compliance with legal weights-and-measures regulations guarantees good metering accuracy at low pressure, is of the order of 20 to 30 for volume-measuring meters. Such dynamic ranges are sometimes too small to cover all of the flow rates applicable to certain public distribution stations, or certain industrial customers. This leads to low flow rates being metered poorly. Furthermore, the rules governing dimensioning of delivery stations generally lead to expander-regulators being installed that are of a capacity that is very much greater than that of the meters, under normal operating conditions. Consequently, in certain situations, there is a risk of the meter being damaged by its maximum flow rate being temporarily exceeded.
Proposals have also been made, in particular in document EP-A-0 337 887, for a multifunction integrated expansion station for feeding gas to a secondary network. In that case, the expander-regulator 1C, a straightener element 8C, a meter 2C, and a flowrate limiter 9C (FIG. 4) are all incorporated in a single outer body extending over a distance 5C and connected firstly to an upstream pipe 6C in which there obtains an upstream pressure Pe, and secondly to a downstream pipe 7C in which there obtains a regulated downstream pressure Pa. The link 4C enables a control signal to be applied to the expander-regulator 1C representing the downstream pressure Pa. Metering is performed at a variable metering pressure Pv which differs from the regulated downstream pressure, given the presence of the flowrate limiter 9C which develops headloss. This leads to the dynamic range of the meter 2C being increased, while also protecting the meter against possible excess flowrate.
A variable pressure meter of that type is advantageous because of the increase in dynamic range that it makes possible and because of its compactness. Nevertheless, it must be specially designed since the various elements of the expansion station must be integrated in a common housing. This leads in particular to a special and relatively complex design for the flow straightener 8C and the limiter 9C, in particular.